1. Field of the invention
The present invention relates generally to absorbent solutions for use with absorption refrigeration apparatus and, more particularly, to absorbent solutions which include water as refrigerant and which do not undergo crystallization even at low temperatures.
2. Description of the Related Art
Typical absorbent solutions for use with absorption refrigeration apparatus include a refrigerant and an absorbent dissolved therein. The principal operation cycle of such an absorption refrigeration apparatus comprises the steps of: heating an absorbent solution whose refrigerant contains the absorbent at a low concentration (hereinafter referred to as a "low-concentration absorbent solution") to evaporate part of the refrigerant, thereby forming an absorbent solution whose refrigerant contains the absorbent at a high concentration (hereinafter referred to as a "high-concentration absorbent solution"); condensing the evaporated refrigerant to form a liquid refrigerant; re-evaporating the liquid refrigerant to remove the latent heat due to the evaporation to the exterior thereby effecting cooling; causing the high-concentration absorbent solution to absorb the re-evaporated refrigerant to form a low-concentration absorbent solution; and removing the resultant absorption heat to the exterior
The concentration of the low-concentration absorbent solution is determined depending upon the re-evaporation temperature of the refrigerant and conditions under which the absorption heat is removed. The concentration of the high-concentration absorbent solution is determined on condition that the absorbent does not undergo crystallization during the cycle of operation.
Heretofore, typical absorbent solutions include water as a refrigerant and lithium bromide as an absorbent. Such a conventional type of lithium bromide-water absorbent solution has involved the following problems The crystallization temperature of lithium bromide is so low that the concentration of the high-concentration absorbent solution is limited to a low level, and it is impossible to assure a sufficiently large concentration difference between the low-concentration absorbent solution and the high-concentration absorbent solution which difference is determined on the basis of cooling conditions such as the re-evaporation temperature of the refrigerant and the temperature of the absorption heat. This makes it necessary to increase the quantity of absorbent solution to be circulated, with the result that a result coefficient (a quantity Q.sub.E of heat absorbed in an evaporator/a quantity Q.sub.G of heat generated in a generator) is lowered and therefore cooling efficiency is limited. Even if the concentration of the absorbent is maintained at a level at which no crystallization takes plate in the range of operating temperatures, the risk of crystallization will become great when the temperature of the absorbent solution falls to ambient air temperature after the stoppage of the refrigeration apparatus. Accordingly, it has been dangerous to operate the refrigeration cycle under such a condition.
As will be evident from the foregoing, if the lithium bromide-water solution is used as an absorbent solution in an absorption refrigeration apparatus, the lithium bromide-water absorbent solution undergoes crystallization in an absorber when the concentration of the absorbent solution reaches a high level of 63.5 weight percent under the conditions of an absorbent solution temperature of 50.degree. C. at a refrigerant evaporation temperature of 5.degree. C. (in the case of a dual-effect absorption refrigeration apparatus, when the concentration reaches 66.5 weight percent). It is, therefore, impossible to use the above-described lithium bromide-water solution in such an absorption refrigeration apparatus. Also, if an absorbent solution which is maintained at a low concentration is made to absorb a vapor refrigerant at a low temperature in order to prevent the occurrence of crystallization, this means that the low-temperature absorbent solution is cooled and the temperature difference between cool air and ambient air (35.degree. C.) becomes small. As a result, the cooling efficiency of cooling the absorbent solution lowers or the absorbent solution becomes impossible to cool. On the other hand, although water-cooled systems have been proposed, they require additional cool water supplying equipment, and hence their applications are limited in terms of installation costs and installation places. In addition, the water-cooled systems are not suitable for home use in views of the cost of cool water and the saving of water.
An investigation was made of some other types of absorbent solution in order to overcome the above-described disadvantages of the lithium bromide-water system and to enlarge the temperature difference between the evaporator and the absorber. An aqueous solution containing a system comprised of the lithium bromide-water system, zinc bromide and zinc chloride exhibited acidity and an extremely strong corrosive action. In a diluted one of this aqueous solution (10 weight percent or less), precipitates were produced due to the generation of zinc hydroxide. An aqueous solution containing a system comprised of the lithium bromide-water system and calcium bromide exhibited a strong corrosive action, and involved the disadvantage that precipitates were produced due to the addition of lithium hydroxide as an anti-corrosive agent. Although other components such as lithium thiocyanate and ethylene glycol were researched, they were not suitable for practical use because of their inferior heat resistance.
As described above, none of the lithium bromide-water absorbent solutions have been satisfactorily used with absorption refrigeration apparatus.